1. Field of the Invention
The present invention relates to a carbonaceous electrode material for a secondary battery, more particularly a carbonaceous material suitable as an electrode material for a high-energy density non-aqueous solvent-type secondary battery, and a process for production thereof. The present invention also relates to an electrode structure comprising such a carbonaceous electrode material, and a non-aqueous solvent-type secondary battery having such an electrode structure.
2. Description of the Related Art
Accompanying the development of, e.g., video tape recorders and small-sized communication appliances reduced in size and weight, there has been an increasing demand for a secondary battery of a high energy density as a power supply for such appliances. Non-aqueous solvent-type lithium secondary batteries have been proposed therefor (e.g., Japanese Laid-Open Patent Application (JP-A) 57-208079, JP-A 62-90863, JP-A 62-122066 and JP-A 2-66856). These batteries use a negative electrode comprising a carbonaceous material doped with lithium instead an electrode of lithium metal so as to alleviate the danger of internal short circuit due to occurrence of dendorite and improve the charge-discharge characteristic, storage stability, etc.
In order to produce a battery of a high energy density, it is important that the carbonaceous material constituting the negative electrode can be doped and de-doped with a large amount of lithium. In order to provide a high-energy density per unit volume of a battery, it is important to use a carbonaceous material having a large capacity of doping and dedoping (liberation) of an active substance (i.e., lithium) and fill the negative electrode with as large a quantity as possible of the carbonaceous material.
In the above-mentioned prior proposals, it has been proposed to use graphite or a carbonaceous material obtained by carbonizing an organic material as a negative electrode material for non-aqueous solvent-type lithium secondary batteries.
Graphite has a large true density of 2.27 g/cm.sup.3, and this is advantageous in filling a negative electrode with a large amount of carbonaceous material. When graphite is doped with lithium, a graphite intercalation compound is formed. In this instance, a graphitic material having a larger crystallite size in its c-axis direction is liable to receive a larger strain acting on the crystallites at the time of repetition of doping-dedoping, thus being liable to break the crystalline structure. Accordingly, a secondary battery prepared by using graphite or a carbonaceous material having a developed graphite structure is liable to have an inferior charge-discharge repetition performance. Further, in a battery prepared by using such a material having a developed graphite structure, the electrolyte is liable to decompose during operation of the battery in some cases.
On the other hand, a so-called amorphous carbonaceous material as obtained by carbonizing phenolic resin or furan resin can exhibit a high doping-dedoping capacity per unit weight but contains a small weight of carbonaceous material per unit volume because of a small true density on the order of 1.5 g/cm.sup.3. As a result, a secondary battery prepared by constituting the negative electrode with such a carbonaceous material cannot necessarily have a high energy density per unit volume. Further, lithium having doped a negative electrode of such an amorphous carbonaceous material is liable to be not completely dedoped (liberated) but remain in a substantial amount, so that lithium as the active substance is liable to be wasted.